The long-term objective of the proposed research is to elucidate the mechanisms by which cell proliferation and differentiation are controlled. Specifically, the research in this grant investigates the function and regulation of the Retinoblastoma (RB) family of proteins during development. The RB family is thought to play important roles in both cell proliferation and in cell differentiation. Understanding the normal function of this particular family of proteins will enhance our general understanding of how cell proliferation and differentiation are controlled. The proposed studies focus on the RB family homolog in a Drosophila genetic system. Although the RB-E2F pathway has been extensively studied using biochemical approaches, studies of the in vivo function of RB, E2F, and DP family of proteins and their functional interactions have been hampered by the presence of large number of proteins in each family and the lack of efficient approaches to functionally identify genes in the pathway. Studies of the RB pathway in Drosophila circumvents these problems and complements standard biochemical approaches. The much simpler picture of the RB pathway in Drosophila and the ease of combining genetic, molecular, cellular, and developmental approaches in such studies make Drosophila an ideal model system to carry out the proposed research. In preliminary studies we have cloned a Drosophila RB family homolog RBF and have demonstrated that the function and regulation of the RB-E2F pathway is conserved between Drosophila and mammals. Furthermore we have generated a number of reagents for the proposed research including: RBF mutants, monoclonal antibodies against RBF, and a number of RBF overexpression strains. In addition, genetic screens have been set up to identify genes in the RB pathway. In this grant we propose to use this Drosophila genetic system to study the in vivo function of RBF and to obtain a functional dissection of the RB pathway. Specifically, we propose (i) to study the function of RBF during development, (ii) to analyze the structure and function of RBF through rescuing the lethality of RBF mutant, (iii) to investigate the functional relationship between E2F and RBF, and between Dacapo and RBF, (iv) to continue our genetic screen to identify genes of the RB pathway.